Quasistatic Magnetic Field Research Articles

On the Field-Induced Transport of Magnetic Nanoparticles in Incompressible Flow: Existence of Global Solutions

We prove global-in-time existence of weak solutions to a pde-model for the motion of dilute superparamagnetic nanoparticles in fluids influenced by quasi-stationary magnetic fields. This model has recently been derived in Grün and Weiß(On the field-induced transport of magnetic nanoparticles in incompressible flow: modeling and numerics, Mathematical Models and Methods in the Applied Sciences, in press). It couples evolution equations for particle density and magnetization to the hydrodynamic and magnetostatic equations. Suggested by physical arguments, we consider no-flux-type boundary conditions for the magnetization equation which entails H({text {div}},{text {curl}})-regularity for magnetization and magnetic field. By a subtle approximation procedure, we nevertheless succeed to give a meaning to the Kelvin force (mathbf {m}cdot nabla )mathbf {h} and to establish existence of solutions in the sense of distributions in two space dimensions. For the three-dimensional case, we suggest two regularizations of the system which each guarantee existence of solutions, too.

Dynamics and Emission of Relativistic Magnetic Dipoles of a Laser Cluster Plasma

It has been shown that the irradiation of a gas target with nanoclusters by a short (tens of femtoseconds), relativistically intense, circularly polarized laser pulse results in the appearance of magnetic dipoles, which are oriented in the direction of the propagation of the laser pulse and produce a quasi-static regular magnetic field with an amplitude to tenths of the field of the initial laser pulse. At a sufficient concentration of clusters, after the end of the laser pulse, the magnetic moments of the dipoles undergo vibrations and the magnetic field oscillates, which is accompanied by the appearance of a short (few-cycle) secondary terahertz radiation. The condition for the appearance of such a short terahertz pulse is that the cyclotron frequency of an electron on average over the volume of the magnetic field is higher than the inverse lifetime of the cluster.

Robotic Localization Based on Planar Cable Robot and Hall Sensor Array Applied to Magnetic Capsule Endoscope.

Recently an active locomotive capsule endoscope (CE) for diagnosis and treatment in the digestive system has been widely studied. However, real-time localization to achieve precise feedback control and record suspicious positioning in the intestine is still challenging owing to the limitation of capsule size, relatively large diagnostic volume, and compatibility of other devices in clinical site. To address this issue, we present a novel robotic localization sensing methodology based on the kinematics of a planar cable driven parallel robot (CDPR) and measurements of the quasistatic magnetic field of a Hall effect sensor (HES) array. The arrangement of HES and the Levenberg-Marquardt (LM) algorithm are applied to estimate the position of the permanent magnet (PM) in the CE, and the planar CDPR is incorporated to follow the PM in the CE. By tracking control of the planar CDPR, the position of PM in any arbitrary position can be obtained through robot forward kinematics with respect to the global coordinates at the bedside. The experimental results show that the root mean square error (RMSE) for the estimated position value of PM was less than 1.13 mm in the X, Y, and Z directions and less than 1.14° in the and orientation, where the sensing space could be extended to ±70 mm for the given 34 × 34 mm2 HES array and the average moving distance in the Z-direction is 40 ± 2.42 mm. The proposed method of the robotic sensing with HES and CDPR may advance the sensing space expansion technology by utilizing the provided single sensor module of limited sensible volume.

Design of Multi-Degrees-of-Freedom Microrobots Driven by Homogeneous Quasi-Static Magnetic Fields

Wireless robots at the subcentimeter size are often actuated using externally generated magnetic fields. For most applications, these remote magnetic microrobots are located relatively far from the magnetic field generation sources. In this condition, all microrobots receive approximately the same driving magnetic field (which we term a homogeneous field). While some solutions have been presented to allow for the creation of simple onboard tools, the full potential of the homogeneous magnetic field for multi-degrees-of-freedom (DOF) actuation has not been exploited. Here we introduce a design framework to utilize the maximum number of independently controlled DOFs on a microrobot system. We make use of three classes of mechanisms which are commonly used in practice and allow for more complex microrobots with up to eight DOFs. To verify the functionality of our framework, we used it to design an optimized drug delivery robot equipped with a 3-DOF drug-releasing mechanism and a 4-DOF motion mechanism. Experiments are performed to actuate each one of the robot's seven DOFs individually, where the cross-talk error between these seven DOFs averaged 7% with a max error of 18.3%.

Generation of a quasi-static magnetic field by a circularly polarised laser pulse due to tunnelling gas ionisation

We present a theoretical model of the quasi-static magnetic field generation in a laser channel, which is formed behind the front of a short laser pulse that ionises a gas. The generation of a magnetic field is caused by the appearance of the electron pressure anisotropy during tunnelling ionisation of atoms. In the considered case of subrelativistic laser light intensities, the generated magnetic field can reach ∼1 MG with an energy transformation ratio of about 1 %, which paves the way for identifying the proposed mechanism when use is made of a wide class of ultrashort pulse lasers.

A flexible and noncontact guided-wave transducer based on coils-only EMAT for pipe inspection

Ultrasonic guided wave testing technique, with its advantages of long propagation distance, low attenuation and high testing efficiency, has been widely used in nondestructive testing and structure health monitoring for pipes. In this paper, a flexible and noncontact guided-wave transducer based on a novel and coils-only electromagnetic acoustic transducer (EMAT) is proposed. In this new transducer, a planar magnetic coil of racetrack shape fed with rectangular-pulsed current is used to produce a quasi-static magnetic field, while a planar meander coil fed with RF-pulse current is used for pulsing and receiving guided-wave signals. Based on the magnetostrictive mechanism, the proposed transducer can generate and receive guided wave of L(0,2) mode in ferromagnetic metal pipes without coupling medium. Electromagnetic fields for the two coils and the induced ultrasonic waves are studied by simulation. Both the simulation and experimental results show that the proposed transducer is effective for generating and receiving ultrasonic guided wave of L(0,2) mode in ferromagnetic pipes, and can be applied on detection for corrosion. Due to the flexible, noncontact feature, and compact configuration of the proposed transducer, it can be conveniently applied for pipes in different diameter with rusty surface or coating in narrow space.

Laser-triggered stochastic volumetric heating of sub-microwire array target

The interaction of an ultrashort laser pulse of moderate intensity ( ≳ 1018 W/cm2) with an innovative high-average-density target consisting of numerous sub-microwires was examined using 3D PIC simulations. We geometrically optimize the target to increase the absorption of laser light and energy (temperature) and the number of hot electrons. To explain the production of super-ponderomotive electrons (with energies significantly exceeding the ponderomotive energy), we use the test particle approach and stochastic dynamics methods to study the electron dynamics in a complicated EM field consisting of incident and reflected laser waves, the Coulomb field of charge separation at wire–vacuum interfaces, and the quasistatic magnetic field. The research demonstrates the stochastic nature of high-energy electron production in such a complicated EM field using Lyapunov exponent analysis. We describe the practical fabrication of such targets in the context of possible experiments. The considered laser–target design has a potential for use in compact laser-based neutron, x-ray sources.

Direct laser acceleration of electrons assisted by strong laser-driven azimuthal plasma magnetic fields.

A high-intensity laser beam propagating through a dense plasma drives a strong current that robustly sustains a strong quasistatic azimuthal magnetic field. The laser field efficiently accelerates electrons in such a field that confines the transverse motion and deflects the electrons in the forward direction. Its advantage is a threshold rather than resonant behavior, accelerating electrons to high energies for sufficiently strong laser-driven currents. We study the electron dynamics via a test-electron model, specifically deriving the corresponding critical current density. We confirm the model's predictions by numerical simulations, indicating energy gains two orders of magnitude higher than achievable without the magnetic field.

Generation and regulation of electron vortices in an underdense plasma by Laguerre-Gaussian laser pulses

An analytical model is presented to control the angular momentum of electrons by impinging Laguerre-Gaussian (LG) laser beam in an underdense warm plasma. During intense laser-plasma interaction, radiative effects of intense long laser pulses become important, which are analyzed under quasi-static approximation. It is shown that the spot size of the laser beam controls the depth of doughnut-shaped ion-channel created in the plasma. Due to inhomogeneity of plasma density, the nonlinear beating of two fast-time-scale laser fields results in the generation of slow-time-scale “dc” magnetic fields. These fields confine the fast electrons in relativistic laser channel and support their direct coupling with the lasers’ fields. Inside the doughnut-shaped channel, the electrons are found to execute transverse betatron oscillations. At resonance, the laser field efficiently transfers the energy and momentum to the plasma electrons which can be controlled by the lasers and plasma parameters. The angular momentum of the electrons is also found to be dependent on the magnitude of the quasi-static axial magnetic field. Moreover, regulation of the electron angular momentum can explore various interesting areas including coherent vortex beam generation via synchrotron radiation process.

Micromagnetic Investigation of Magnetization Reversal in Sphere-Shaped Ferromagnetic Nanoparticle

In this paper, the magnetization reversal of sphere-shaped ferromagnetic nanoparticles has been investigated by means of micromagnetic simulation. Some ferromagnetic particles such as Cobalt, Iron, Nickel, and Permalloy were modeled with size variation from 50 nm to 100 nm. The discretization of the ferromagnetic model was used a cell size of 2.5×2.5×2.5 nm3 considering the exchange length (lex) of the materials. The quasi-static magnetic field was induced into the nanosphere to observe the magnetization response under time dependence. It is found that the coercivity values are decreased as the sphere size increased, which was conformed the experimental results. It is also observed that the domain structure of a single particle in remanent and ground-state condition are identical. Therefore, the specific understanding of magnetization process and domain structures in ferromagnetic nanoparticles could be an important step in the development of nanopatterned magnetic memory storage.

The diameter effect on the magnetization switching time of sphere-shaped ferromagnets using micromagnetic approach

In this work, the magnetization switching time has been studied as the diameter variation of sphere-shaped ferromagnets by means of micromagnetic simulation. Some ferromagnetic elements such as cobalt, iron, nickel, and permalloy were numerically simulated with diameter size variation from 50 nm to 100 nm. The micromagnetic simulation was performed by public software Object Oriented Micromagnetic Framework (OOMMF) based on the Landau-Lifshitz-Gilbert (LLG) equation. The ferromagnetic nano-sphere was induced by the quasi-static magnetic field to observe its magnetization response. Generally, it is observed that the switching time increases as the diameter size increases on the ferromagnetic elements. However, the switching time are relatively insensitive as the diameter increases in the Cobalt element for the diameter range from 60 nm to 90 nm. This behavior is contributed to the distribution of magnetization easy axis on the ferromagnetic elements. The understanding of domain structures during magnetization switching process is important in the development of nano-patterned magnetic memory storage.

Magneto-optical spectroscopy with arbitrarily polarized intensity-modulated light in He4 atoms

We present both a theoretical and experimental study of the magneto-optical spectra induced with intensity-modulated resonant laser light that synchronously pumps and detects the metastable $^{4}\mathrm{He}$ atoms in the presence of a quasistatic magnetic field. We extend previous work and derive complete analytical expressions for the resonance signals by taking into account arbitrary magnetic-field directions and arbitrary light polarizations. The analytical results are derived by solving the Liouville equation using the irreducible tensor formalism on the assumption of low light power. We discuss the potential application of the derived analytical results in constructing and optimizing a dead-zone-free all-optical atomic magnetometer.

Uniform warm dense matter formed by direct laser heating in the presence of external magnetic fields.

With the recent realization of kilotesla quasistatic magnetic fields, the interaction of a laser with magnetized solids enters an unexplored new regime. In particular, a circularly polarized (CP) laser pulse may propagate in a highly magnetized plasma of any high density without encountering cutoff reflection in the whistler mode. With this, we propose a scheme for producing uniform warm dense matter (WDM) by direct laser heating with a CP laser irradiating onto the target along the magnetic field. It is shown by particle-in-cell simulations, which include advanced ionization dynamics and collision dynamics, moderately intense right-hand CP laser light at 10^{15}W/cm^{2} can propagate in solid aluminum and heat it efficiently to the 100eV level within picoseconds. By using two laser pulses irradiating from two sides of a thin solid target, uniform heating to WDM can be achieved. This provides a controllable way to create WDM at different temperatures.

Gain of electron orbital angular momentum in a direct laser acceleration process.

Three-dimensional "particle in cell" simulations show that a quasistatic magnetic field can be generated in a plasma irradiated by a linearly polarized Laguerre-Gauss beam with a nonzero orbital angular momentum (OAM). Perturbative analysis of the electron dynamics in the low intensity limit and detailed numerical analysis predict a laser to electrons OAM transfer. Plasma electrons gain angular velocity thanks to the dephasing process induced by the combined action of the ponderomotive force and the laser induced-radial oscillation. Similar to the "direct laser acceleration," where Gaussian laser beams transmit part of its axial momentum to electrons, Laguerre-Gaussian beams transfer a part of their orbital angular momentum to electrons through the dephasing process.

High magnetic shielding properties of an MgB2 cup obtained by machining a spark-plasma-sintered bulk cylinder

Superconductors are key materials for shielding quasi-static magnetic fields. In this work, we investigated the shielding properties of an MgB2 cup-shaped shield with small aspect-ratio of height/outer radius. Shape and aspect-ratio were chosen in order to address practical requirements of both high shielding factors (SFs) and space-saving solutions. To obtain large critical current densities (Jc), which are crucial for achieving high magnetic-mitigation performance, a high-purity starting MgB2 powder was selected. Then, processing of the starting MgB2 powder into high density bulks was performed by spark plasma sintering. The as-obtained material is fully machinable and was shaped into a cup-shield. Assessment of the material by scaling of the pinning force showed a non-trivial pinning behaviour. The MgB2 powder selection was decisive in enlarging the range of external fields where efficient shielding occurs. The shield’s properties were measured in both axial- and transverse-field configurations using Hall probes. Despite a height/outer radius aspect ratio of 2.2, shielding factors higher than 104 at T = 20 K up to a threshold field of 1.8 T were measured in axial-field geometry at a distance of 1 mm from the closed extremity of the cup, while SFs > 102 occurred in the inner half of the cup. As expected, this threshold field decreased with increased temperature, but SFs still exceeding the above mentioned values were found up to 0.35 T at 35 K. The shield’s shape limits the SF values achievable in transverse-field configuration. Nevertheless, the in-field Jc of the sample supported SFs over 40 at T = 20 K up to a field of 0.8 T, 1 mm away from the cup closure.

Magnetoquasistatic resonances of small dielectric objects

A small dielectric object with positive permittivity may resonate when the free-space wavelength is large in comparison with the object dimensions if the permittivity is sufficiently high. We show that these resonances are described by the magnetoquasistatic approximation of the Maxwell's equations in which the normal component of the displacement current density field vanishes on the surface of the particle. They are associated to values of permittivities and frequencies for which source-free quasistatic magnetic fields exist, which are connected to the eigenvalues of a magnetostatic integral operator. We present the general physical properties of magnetoquasistatic resonances in dielectrics with arbitrary shape. They arise from the interplay between the polarization energy stored in the dielectric and the energy stored in the magnetic field. Our findings improve the understanding of resonances in high-permittivity dielectric objects and provide a powerful tool that greatly simplifies the analysis and design of high index resonators.

Comment on “Electric Power Generation from Earth’s Rotation Through its own Magnetic Field”

In a recent article [Phys. Rev. Applied 6, 014017 (2016)], Chyba and Hand propose a new scheme to generate electric power continuously at the expense of Earth's kinetic energy of rotation, by using an appropriately shaped cylindrical shell of a well chosen conducting ferrite, rigidly attached to the Earth. No experimental confirmation is reported for the new prediction. In the present Refutation, I first use today's standard electromagnetism and essentially the same model as Chyba and Hand to show in a very simple way that no device of the proposed type can produce continuous electric power, whatever its configuration or size, in agreement with widespread expectation. Next, I show that the prediction of non-zero continuous power by Chyba and Hand results from a confusion of frames of reference at a critical step of their derivation. When the confusion is clarified, the prediction becomes exactly zero and the article under discussion appears as pointless. At the end, I comment about the persistent invocation by Chyba and Hand of the misleading legacy notion that quasi-static magnetic fields have an intrinsic velocity, and other questionable concepts.

Proton deflectometry of a capacitor coil target along two axes

A developing application of laser-driven currents is the generation of magnetic fields of picosecond–nanosecond duration with magnitudes exceeding $B=10~\text{T}$ . Single-loop and helical coil targets can direct laser-driven discharge currents along wires to generate spatially uniform, quasi-static magnetic fields on the millimetre scale. Here, we present proton deflectometry across two axes of a single-loop coil ranging from 1 to 2 mm in diameter. Comparison with proton tracking simulations shows that measured magnetic fields are the result of kiloampere currents in the coil and electric charges distributed around the coil target. Using this dual-axis platform for proton deflectometry, robust measurements can be made of the evolution of magnetic fields in a capacitor coil target.

Indukcione zavojnice za merenje kvazistacionarnih magnetnih polja niskog intenziteta

Short air core induction coils with a large radius and a large number of turns can be successfully used as induction sensors for detecting low-intensity quasistationary magnetic fields, such as magnetic fields of ships. The application of these sensors is presented in detail, and then a practical methodology for determining all the necessary parameters of the sensor is proposed. The sensitivity of sensors with actual design parameters is determined and inductance is calculated in several ways. The practical results of ship's magnetism measurement and the calculation of all parameters of the coil are presented. A comparative analysis of all results was performed and the observed deviations were thoroughly commented on.

Modeling of electromechanical processes of the linear permanent magnet motor drive of the one-mass vibro-impact system

The paper presents the tubular linear permanent magnet vibratory motor at a vibro-impact operating mode. The model for research of electromechanical processes in the one-mass vibro-impact system with the linear permanent magnet motor drive, which is founded on an equivalent circuit with the lumped parameters, is offered. Electric parameters of the model are dependent on operating frequency and consider magnetic losses in the motor core. Determination of calculation scheme parameters was made on the basis of field calculation by means of a finite element method. Owing to an axial motor symmetry the problem was considered in axisymmetric approach, by means of the equations of a quasi-stationary magnetic field in a time domain that has allowed considering the eddy currents in massive poles of the motor anchor. Determination of magnetic losses was made experimentally, by means of a prototype of the linear permanent magnet vibratory motor. Mechanical equivalent circuit parameters were determined experimentally, by means of the damped oscillation method and with the help of direct weighing of construction elements. We applied the Hertz’s formula for modeling of an impact force. The solution of the system dynamic equations was made by means of structural modeling in the Simulink/Matlab software package. Furthermore, we calculated characteristics of the one-mass electromechanical system in dependence of the impact parameters and operating frequency. For the validation of the calculation model, experimental studies of electromechanical characteristics of system with use of a prototype of the linear permanent magnet motor, which works in vibro-impact mode, are executed. Comparative calculation by means of the developed model is executed.